Semiconductor nanoparticles have drawn attention since they emit strong fluorescence with a narrow full width at half maximum (FWHM). Thus, various fluorescent colors can be created, and their future applications can be nearly infinite.
Semiconductor nanoparticles having particle sizes of 10 nm or less are located in the transition region between bulk semiconductor crystals and molecules. Their physicochemical properties are therefore different from both bulk semiconductor crystals and molecules. In this region, the degeneration of the energy band that is observed in bulk semiconductors is removed and the orbits are dispersed. Due to this quantum-size effect, the energy gap of semiconductor nanoparticles increases or decreases as their particle sizes decrease or increase, respectively. The varying energy gap affects the fluorescent properties of the nanoparticles. More specifically, nanoparticles having small particle sizes and a large energy gap emit fluorescence at shorter wavelengths, and in contrast, those having large particle sizes and a small energy gap emit fluorescence at longer wavelengths. Accordingly, particle size control of semiconductor nanoparticles enables the development of various materials which emit fluorescence with a desired color.
To use semiconductor nanoparticles as fluorescent materials, their particle sizes must be controlled. Control of particle-size distribution to yield monodisperse nanoparticles enables semiconductor nanoparticles which have suitable fluorescent properties and exhibit spectrum with a narrow full width at half maximum.
Methods for producing semiconductor nanoparticles basically comprise the steps of preparing nanoparticles and narrowing their particle-size distribution to yield monodisperse nanoparticles. Semiconductor nanoparticles can be easily prepared by dissolving equimolar amounts of precursors of Cd and X, wherein X is S, Se or Te. This is also true for the production of, for example, CdSe, ZnS, ZnSe, HgS, HgSe, PbS or PbSe. The semiconductor nanoparticles prepared by the above method exhibit a wide distribution of particle size. Attempts have been made to attain a monodisperse distribution by using chemical techniques to precisely separate and extract only the semiconductor nanoparticles of a specific particle size from semiconductor nanoparticles having a wide distribution of particle sizes immediately after preparation. The attempts to attain a monodispersed distribution of particle sizes that have been reported so far include: separation by electrophoresis that utilizes variation in the surface charge of nanoparticles depending on their particle sizes; exclusion chromatography that utilizes differences in retention time due to different particle sizes; and size-selective precipitation that utilizes differences in dispersibility in an organic solvent due to differences in particle sizes.
The aforementioned production methods are carried out by preparing semiconductor nanoparticles having a wide distribution of particle sizes and then regulating and selecting the particles sizes. In contrast, methods in which preparation of nanoparticles and regulation of particle size to attain a monodisperse distribution are performed in one step have been reported. An example of these methods is the reversed micelle method. In this method, amphiphilic molecules such as sodium diisooctyl sulfosuccinate and water are dispersed in an organic solvent such as heptane, reversed micelles are formed in the organic solvent, and precursors are reacted in the aqueous phase alone in the reversed micelles. The inside of the reversed micelle is regarded as a reaction field, and the size of the reaction field is regulated by controlling the quantitative ratio of the amphiphilic molecules to water, thereby sorting nanoparticles into uniform particle sizes. The sizes of the resulting semiconductor nanoparticles depend on the sizes of reversed micelles and semiconductor nanoparticles having a relatively narrow distribution of particles sizes can be produced. Separately, a method for preparing nanoparticles and regulating particle sizes to attain a monodisperse distribution in one step with the use of Ostwald ripening has been reported. This method, however, requires preparation of a highly toxic reagent at high temperatures and thus exhibits insufficient safety.
Another possible solution than the above methods is size-selective photoetching utilizing photocatalytic reactions. The method utilizes the oxidative dissolution of a metal chalcogenide semiconductor in the presence of dissolved oxygen when irradiated with light to thereby yield monodisperse nanoparticles. For examples, upon photoexcitation of CdS nanoparticles in the presence of dissolved oxygen, excited electrons progress reduction of oxygen, and holes progress oxidation of the CdS nanoparticles. These photocatalytic reactions proceed during excitation of semiconductor nanoparticles. Dissolution of all the excited semiconductor nanoparticles terminates at a particle size having an energy gap corresponding to the energy of irradiated light at the shortest wavelength. More specifically, in size-selective photoetching, semiconductor nanoparticles having a wide distribution of particle sizes are irradiated with light at a shorter wavelength than the wavelength of their absorption edge to thereby selectively dissolve and optically pump semiconductor nanoparticles having large particle sizes. The resulting semiconductor nanoparticles have smaller and more uniform particle sizes. By selecting the wavelength of irradiated light, monodisperse nanoparticles having an arbitrary particle size can be prepared at room temperature with relatively high safety. In addition, the use of monochromatic light yields nanoparticles having a narrower and finely adjusted particle-size distribution. Preparation of monodisperse semiconductor nanoparticles by size-selective photoetching is described typically in J. Electrochem. Soc. 145:1964(1998); and J. Pys. Chem. B. 105:6838(2001). The semiconductor nanoparticles exhibit deviations in terms of root-mean-square (rms) of 15% or more of the average particle size before light irradiation. When they are irradiated with light at a wavelength of 476.5 nm, these semiconductor nanoparticles exhibit a very narrow distribution of particle sizes, i.e., the deviation in terms of rms is approximately 6% of the average particle size. This indicates that the distribution of particle sizes is very close to the monodispersed state.
In the conventional size-selective photoetching methods as described in the above literature, monodisperse semiconductor nanoparticles are prepared in the following manner.
Initially, an aqueous solution (1000 ml) of sodium hexametaphosphate (0.1 mmol) and cadmium perchlorate (0.2 mmol) is prepared and is adjusted to pH 10.3. Hydrogen sulfide gas (0.2 mmol) is then injected into the solution while vigorously stirring by bubbling with nitrogen gas, followed by stirring for a while. The solution changes its color from optically transparent colorless to optically transparent yellow in this procedure. The resulting semiconductor nanoparticles having a wide particle-size distribution and being stabilized by hexametaphosphoric acid are then subjected to size-selective photoetching to thereby have a monodisperse distribution. Initially, the solution of the semiconductor nanoparticles is bubbled with nitrogen gas, followed by bubbling with oxygen gas for 10 minutes. Methylbiologen (50 μmol/l) is then added to the solution, and light is applied to the solution while stirring.
However, the present inventors have found that such semiconductor nanoparticles having a monodisperse particle-size distribution prepared by size-selective photoetching show large variation in their properties. The variations occur typically in surface modification of the semiconductor nanoparticles. Surface modification of semiconductor nanoparticles will be briefly illustrated below.
The semiconductor nanoparticles emit band gap fluorescence derived from inside the semiconductor nanoparticles, as well as another fluorescence which may be derived from an energy level in the energy band inside the semiconductor nanoparticles. The energy level for emitting the latter fluorescence is speculated to be present predominantly in the surface sites of the semiconductor nanoparticles. Control of particle sizes of the semiconductor nanoparticles affects the properties of the band gap fluorescence. The presence of the other fluorescence will deteriorate the properties of such semiconductor nanoparticles having a narrow particle-size distribution and should be avoided. The fluorescence in question can be prevented by modifying the surfaces of the semiconductor nanoparticles to thereby eliminate the energy level derived from the surfaces. Thus, the semiconductor nanoparticles become to emit band gap fluorescence derived from inside thereof alone. For surface modification, attempts have been made to cover the core semiconductor material particles with a semiconductor material, inorganic material or organic material having a band gap larger than the semiconductor material to thereby reduce the fluorescence. The present inventors filed a Japanese patent application on a method for modifying the surfaces of semiconductor nanoparticles by applying electron donating groups to the surfaces.
When semiconductor nanoparticles prepared by size-selective photoetching are subjected to surface modification, the nanoparticles show largely varying fluorescent properties depending on the properties of particles before surface modification. Specifically, the properties of prepared nanoparticles must be uniformized to yield semiconductor nanoparticles having fluorescent properties with a high degree of reproducibility.